Transcriptional regulation is a major event in cell differentiation, proliferation, and apoptosis. Transcriptional activation of a set of genes determines cell destination and for this reason transcription is tightly regulated by a variety of factors. One of its regulatory mechanisms involved in the process is an alteration in the tertiary structure of DNA, which affects transcription by modulating the accessibility of transcription factors to their target DNA segments. Nucleosomal integrity is regulated by the acetylation status of the core histones. In a hypoacetylated state, nucleosomes are tightly compacted and thus are nonpermissive for transcription. On the other hand, nucleosomes are relaxed by acetylation of the core histones, with the result being permissiveness to transcription. The acetylation status of the histones is governed by the balance of the activities of histone acetyl transferase (HAT) and histone deacetylase (HDAC). Recently, HDAC inhibitors have been found to arrest growth and induce apoptosis in several types of cancer cells, including colon cancer cells, T-cell lymphoma cells, and erythroleukemic cells. Given that apoptosis is a crucial factor for cancer progression, HDAC inhibitors are promising reagents for cancer therapy as effective inducers of apoptosis (Koyama, Y., et al., Blood 96 (2000) 1490-1495).
Histone deacetylases (HDACs) are the key enzymatic components of multiprotein complexes responsible for deacetylation of lysine residues in histone and nonhistone protein substrates. HDACs can be subdivided into three major classes according to their sequence homology to the yeast HDACs, Rpd3, Hda1, and Sir2. The class I HDACs (HDACs 1, 2, 3, and 8), homologous to Rpd3, localize primarily in the nucleus and appear to be expressed in most tissues. The class II HDACs (HDACs 4, 5, 6, 7, 9, 10), homologous to Hda1, are able to shuttle between the nucleus and the cytoplasm depending on a variety of regulatory signals and cellular state, and have tissue-specific expression patterns. These HDACs can be further subdivided into class IIa (HDACs 4, 5, 7, 9), and class IIb (HDACs 6, 10). The class III HDACs, homologous to Sir2, are NAD+-dependent deacetylases that are mechanistically distinct from the class I and II HDACs and are not inhibited by classical HDAC inhibitors such as trichostatin A, trapoxin B, or SNDX-275. The HDACs can thus be divided into three classes on the basis of sequence similarity, cellular localization tendencies, tissue expression patterns, and enzymatic mechanism.
The class I HDACs in particular have been closely associated with antiproliferative effects against tumor cells. For example, pharmacological inhibition of HDACs 1-3 leads to induction of the cyclin-dependent kinase inhibitor p21 and concommitant cell cycle arrest. The class IIa HDACs are known to associate with the HDAC3/SMRT/N-CoR complex and MEF2 and as such have important roles in regulating muscle cell gene expression (reviewed in Oncogene 2007, 26, 5450-5467) and the immune response (Biochemical Pharmacology 2007, 74, 465-476). Due to their specific antiproliferative function, selective inhibition of the class I HDACs may be desirable to achieve antitumor efficacy with lower toxicity.
The compounds of the present invention show enhanced potency toward class I HDACs and enhanced antiproliferative efficacy versus cancer cells as compared to SNDX-275, a structurally-related HDAC inhibitor in clinical trials. Class I HDAC inhibitory potency is evaluated by a reporter gene assay that evaluates HDAC subtype activity in the context of relevant multiprotein complexes present in the cell that are typically absent in enzyme activity assays. Thus, the compounds of the present invention possess in-cell inhibitory potency toward class I HDACs that correlates to their improved anticancer efficacy in comparison to SNDX-275.
WO 2007/100657 describes related but structurally different o-phenylendiamine derivatives as cell differentiation inducers. The same type of compounds is also the subject of WO2007/087130. The compounds described in these applications are exclusively o-phenylene derivatives monoacylated with derivatives of benzoic acid. However there remains a need for new compounds with improved therapeutic properties, such as enhanced activity, decreased toxicity, better solubility and/or improved pharmacokinetic profile, to name only a few.
Monoacylated o-phenylendiamines are known in the art as precursors for the preparation of the corresponding benzimidazoles, such preparation methods are e.g. described in DE-A 2 062 265; FR 2 167 954; Rastogi, R., and Sharma, S., Indian J. Chem., Sect. B, 21B (5) (1982) 485-487; Moll, R., et al., Z. Chem. 17 (1977) 133-134; and Hassan, H., et al., Indian J. Chem. 39B (2000) 764-768.
It has been found that the compounds of the present invention are HDAC inhibitors which have anti-proliferative and differentiation-inducing activity, which results in inhibition of tumor cell proliferation, induction of apoptosis and inhibition of invasion. These compounds are therefore useful for the treatment of diseases such as cancer in humans or animals.